The present invention relates to concrete railroad cross ties or sleepers and, more particularly, to railroad ties having rail seats provided with improved abrasion resistance.
Since the early 1960's concrete cross ties have generally been used with direct fixings in which the rail rests on a rubber or plastic pad which in turn rests on the moulded concrete top surface of the tie.
If the pad, which is known as a rail or tie pad is omitted or moves out of position it is well known that direct contact of the rail on the tie can result in serious abrasion of the concrete surface. In such a case, the concrete is ground away by the rail leaving a smooth surface and, if the reduction in depth of the concrete does not lead to structural failure, the tie can continue in service provided the original depth of the sleeper is restored with additional rail pads.
A different type of abrasion has been encountered where the surface of the concrete under the rail pad is eroded. The appearance is that the coarse aggregate is largely unaffected but the mortar (sand and cement) fraction is eroded to a depth of several millimetres leaving a rough surface.
This type of abrasion can occur in a period of two years which is very short in relation to the nominal 40-50 year life of a concrete tie. Apart from ultimately affecting the structural strength of the tie, an immediate problem is that the rough surface causes unacceptably rapid wear of the rail pads. This is serious because one function of the rail pad is to provide electrical insulation between the rail and the tie so that track circuiting can be used as part of the signalling system. So far this form of abrasion has only been found in a location where there is a small radius curve in the track, the trains are heavy, the gross tonnage is high and the climate is wet except in winter when temperatures are generally below freezing. However, it is of sufficient importance to have an adverse effect on the market for concrete ties.
Research has been directed at improving the abrasion resistance of the surfaces of the concrete ties immediately under rails. Techniques exist for producing abrasion resistant surfaces on concrete floors but these depend upon work hardening techniques within a few hours of casting the concrete. This method cannot be used on concrete ties because they are cast upside down so that the top surface is in contact with an accurately made steel mould to ensure the tight tolerances on rail seat flatness and other features are satisfied. Another technique is to improve the curing of concrete by preventing premature drying out of the water required to hydrate the cement. This method has shown some improvement in abrasion resistance but not sufficient to ensure satisfactory performance in the most arduous conditions in track.
A surface coating, such as epoxy resin, is used in coal hoppers and other places where abrasion of concrete occurs. This is costly and because of the high loadings and high frequency stress reversals in track there is concern that the coating might become detached from the concrete. If this occurred it would not be practicable to renew the coating.
The composition of the concrete has an effect on abrasion resistance and the use of harder fine aggregate, such as silicon carbide, should improve the resistance to erosion. However, it is impracticable to cast a part of a tie from different concrete during the normal production process and ensure that it remains in the intended part of the tie. To use special concrete throughout would be prohibitive in material cost and, also, capital cost for additional equipment to store and measure the special materials.
It would be possible to cast a cross tie with a recess in the rail seat and fill it at a later stage with a special concrete. As with a surface coating this introduces a risk that the special concrete will, under the influence of high loads, dynamic action and frost action, become separated from the main body of the tie. Additionally there is a high cost in rehandling the tie and filling the recess with special concrete.